“The victims have told us they were terrified and the ordeal has left them badly shaken up.”

The offenders are believed to have fled the scene after being disturbed before ditching clothing in a nearby bin, at the junction of Caunce Street and Selbourne Road. The items were recovered by police although the knife has yet to be found.

Det Sgt Dickinson added: “One of the shopkeepers has armed himself with a chair to protect himself and his brother, the other shopkeeper.

“There has been a stand-off with the offenders, who left when another customer entered the shop. They made off empty handed.”

Police arrested a 24-year-old woman and a 28-year-old man, both from Blackpool, on suspicion of attempted robbery following the offence.